The present invention relates to a control system for an electric motor, and in particular to measurement of current in electric motors and the control of electric motors based on current measurement.
A closed loop current controller for an electric actuator typically takes the form shown in FIG. 1, where, using vector control, a demand voltage VDQ is applied to each phase winding of a motor 10 and the resultant generated currents, iMotor, measured by a current measurement circuit. The measured currents are then used by a current controller 20, typically a PI controller, to control the motor currents to the requested target current. The input to the PI controller is the difference between the target demand current iDQ and the actual measured current. The output of the controller is typically a set of motor demand voltages that are used to determine PWM waveforms that are applied to the drive bridge switches of a motor drive circuit. A signal which gives the electrical position of the motor is also required so that the voltages can be applied at the correct phases at the correct times. In the system of FIG. 1 a position sensor is shown but the present invention is equally applicable to a position sensorless system, where the position signal is estimated from other sensor measurements.
Due to inaccuracies and limitations of the current sensor the measurement process can introduce harmonics into the output signal of the current sensor that are not present in the currents that are being measured. The current controller then reacts to these harmonics in the sensor output to try to remove them, but their removal by the current controller, to give a smooth output from the current sensor, causes the actual currents to contain these harmonics.
Depending on the frequency and amplitude of these unwanted harmonics the torque produced by the motor can be degraded, with the harmonics potentially causing torque ripple and/or acoustic noise.
Typically two methods of phase current measurement are employed:
1. Phase current sensors, where a current measurement device is placed in each of the phases. (For a three phase system it may be that only 2 phases are measured as the 3rd phase can be calculated from the 2 measured phases).
2. Single current sensor, where the current flowing in the DC link is measured at specific points during the PWM duty cycle to allow the current in the 3 phases to be calculated.
FIG. 3 shows the use of a single current sensor 30 on a three phase motor 10. Also shown in this figure are the bridge switches that are opened and closed according to a PWM pattern set by the voltage demand signal.
The harmonics introduced will be different, depending on the phase current measurement method used. The present invention is applicable to any harmonic introduced by measurement inaccuracies.